1. Field of the Invention
The present invention relates to power converters, and more particularly to the control circuit of power converters.
2. Description of the Related Art
Power converters are used for converting an unregulated power source to a constant voltage source. Power converters generally include a transformer having a primary winding and a secondary winding for providing the isolation. The switching devices are connected to the primary winding for controlling the energy transfer from the primary winding to the secondary winding. A higher operating frequency allows a reduced size and weight for power converters. However, the switching losses, the component stresses, and electromagnetic interference (EMI) are the inherent problems. In recent developments, a popular phase-shift scheme for soft switching has been proposed for high frequency power conversion for reducing switching losses. Among them, the full-bridge (FB) quasi-resonant ZVS technique is described in the following: “Constant frequency resonant power converter with zero voltage switching” by Christopher, P. Henze, Ned Mohan, and John G. Hayes, U.S. Pat. No. 4,855,888, Aug. 8, 1989; “Soft-switching PWM converters” by Guichao C. Hua and Fred C. Lee, U.S. Pat. No. 5,442,540, Aug, 15, 1995; “Soft-switched full-bridge converters” by Yungtaek Jang and Milan M. Jovanovic, Mar. 12, 2002. The active clamp techniques are disclosed for the forward ZVS power converters such as: “Double forward converter with soft-PWM switching” by F. Don Tan, U.S. Pat. No. 5,973,939, Oct. 26, 1999; “Active clamp isolated power converter and method of operating thereof” by Simon Fraidlin and Anatoly Polikarpov, U.S. Pat. No. 6,191,960, Feb. 20, 2001. As for the half-bridge (HB) topology, an asymmetrical scheme is developed for ZVS, “Asymmetrical power converter and method of operation thereof” by Rui Liu, U.S. Pat. No. 6,069,798, May 30, 2000. In the various ZVS converters, the parasitic leakage inductance of the transformer or the additional magnetic components are employed as a resonant inductor or switches for generating the circulating current for achieving the zero voltage transition and switching.
FIG. 1 illustrates a traditional active clamp power converter. FIG. 1A˜FIG. 1D illustrate four operational stages of the aforementioned power converter. As FIG. 1A illustrates, a first signal S1 switches on a transistor Q1 to transfer the energy from an input of the power converter to an output of the power converter via a transformer T1. When the transistor Q1 is switched off as illustrated in FIG. 1B, the magnetic energy of the transformer T1 shall flow into the capacitor C1 via a parasitic diode D2. Meanwhile, a second signal S2 shall turn on a transistor Q2 for achieving the soft switching operation of the transistor Q2. After the magnetic energy of the transformer T1 is fully discharged, the capacitor C1 shall start to charge the transformer T1 via the transistor Q2, as illustrated in FIG. 1C. FIG. 1D illustrates the fourth operation stage, in which the second signal S2 turns off the transistor Q2 to cut off the current flowing between the transformer T1 and the capacitor C1. Meanwhile, the energy stored in the transformer T1 shall produce a circulating current to discharge the parasitic capacitor Cj of the transistor Q1. To turn on a parasitic diode D1 for achieving soft switching operation of the transistor Q1, the parasitic capacitor Cj must be fully discharged in advance.
The criterion for achieving the transition is given by:Ip2/(2×Lp)>Cj×VIN2/2
where Lp is the primary-winding inductance of the transformer T1, Ip is the primary-winding current of the transformer, and VIN is the input voltage of the power converter.
Since the resonant frequency fr is given by:fr=1/(2π×Lp×Cj)
A delay time TD1 for achieving the phase shift for soft switching operation is given by:
                              T                      D            ⁢                                                  ⁢            1                          =                ⁢                  1          /                      (                          4              ×                              f                r                                      )                                                  =                ⁢                  π          ×                      L            p                    ×                                    C              j                        /            2                              
FIG. 2 illustrates a traditional asymmetrical half bridge forward power converter, in which the operation of the signals S1 and S2 is the same as the power converter shown in FIG. 1. Although the aforementioned power converters are able to achieve soft switching operation to reduce the switching loss under heavy load conditions, the drawback, however, is higher power consumption under light load conditions.